Alpha-adrenergic regulation of splanchnic volume and cardiac output in the dog

The present study examined whether α-adrenergic stimulation causes a change in splanchnic intravascular volume in the anaesthetized animal with an intact circulation, which region(s) mediate the volume change, and whether the splanchnic volume change influences cardiac output. In order to ascertain that a radionuclide imaging technique could be used to assess total splanchnic volume changes, drugs known to increase or decrease splanchnic volume were infused on 21 occasions in eight dogs studied under conditions of selective perfusion and drainage of the splanchnic vasculature with erythrocytes labelled with ⁹⁹Tc^m and a gamma camera placed over the abdomen. For these 21 infusions, volume and radionuclide count changes were related: r ≥ 0.90 (n = 20), r = 0.76 (n = I). After ascertaining tissue attenuation and blood radioactivity in four of the animals, the standard error for a single estimate of the absolute volume change using the radionuclide technique was determined to be 75 ml. In six animals with intact circulations, phenylephrine (40-80 μg min^-1) for 20 min was associated with an increase in cardiac output of 12 ± 2% (P < 0.001) and a decrease in total splanchnic volume estimate to be 431 ± 95 ml (P < 0.001). The splanchnic volume decrease was due entirely to decreases in splenic and intestinal volume. In eight eviscerated animals, cardiac output decreased by 30 ± 2% (P < 0.001) during phenylephrine. With 40-160 μg phenylephrine boluses in the splenic artery in three dogs, splenic volume decreased by 11 ± 2% (P < 0.001) and cardiac output increased by 18 ± 4% (P < 0.01), but neither parameter changed with left atrial boluses. Thus, α-adrenergic stimulation is associated with a decrease in splanchnic volume in the animal with an intact circulation, and this decrease acts to increase cardiac output.